Statistical Study of the Long-Period Induced Geoelectric Field Caused by Magnetic Storms and Its Implication on Mantle Exploration

Abstract The ground induced geoelectric field caused by magnetic storms is not only a hazard, but also a helpful tool to explore the underground conductivity. Especially, the long period (>105 s, LP) induced field deserves more concerns as it could reach the mantle. However, the occurrence rate and the period range of the LP induced geoelectric field during different magnetic storms are unclear. This statistical work examines the occurrence and the period upper limit in the whole solar cycle 23. The geoelectric field and geomagnetic field measured continuously at Memambetsu observatory in Japan from 1989 to 2008 are studied. The LP electromagnetic field is identified by the wavelet coherence spectra. The results show that the LP induced geoelectric field stably occurred during magnetic storms in the solar cycle. The LP induced field in the first three months of each year is significantly different from that in the three months around super magnetic storms. The longest period of the induced geoelectric field during the magnetic storms is 9x105 s. The period upper limit of the induced δ Ey is larger than that of the induced δEx, which significantly increases with the storm intensity range. The distribution of the LP δEx on the magnetic local time is asymmetric. To quantify the potential application of such a LP electromagnetic field on the mantle conductivity, we check the uncertainty of resistivity from inversion under the condition of plane-wave for layered medium. We set the sources to be the ring current and the field-aligned current in their real scales. As a result, the apparent resistivity is obtained within 10% uncertainty by δEy, and within 20% by δEx.